The present invention relates to a projection type image display apparatus and an image display method, and in particular, relates to a technology for displaying images on a screen by illuminating a spatial light modulator using light from a coherent light source.
As a projection type image display apparatus for displaying images by projecting light onto a screen, various types of apparatuses have been proposed, including an apparatus available on the market, which is a so-called “optical type projector.” The basic principle of such a projection type image display apparatus is to generate an original two-dimensional image by utilizing a spatial light modulator such as a liquid crystal micro-display or a DMD (Digital Micro-mirror Device), and to magnify and project the two-dimensional images on a screen by utilizing a projection optical system.
A general optical type projector adopts a system which illuminates a spatial light modulator such as a liquid crystal display using a white light source such as a high pressure mercury lamp, magnifies a modulated image thus obtained, by means of lenses and projects the same on a screen. For example, Japanese Patent Publication No. 2004-264512A discloses a technology that divides white light generated by a super high pressure mercury lamp into three prime color components of R (red), G (green), and B (blue) by means of a dichroic mirror, leads these lights to respective spatial light modulators for the prime colors, synthesizes modulated images thus generated for each of the prime colors by means of a cross dichroic prism and projects the same on a screen.
However, the service life of a high brightness discharge lamp such as a high pressure mercury lamp is comparatively short, wherein if such a lamp is utilized in an optical type projector, etc., it is necessary to frequently change the lamp. Further, since it is necessary to utilize a comparatively large optical system such as a dichroic mirror, in order to pick up lights of respective prime colors, there is a disadvantage that the entire apparatus becomes large-sized. Therefore, a system which uses a coherent light source such as a laser has been proposed. For example, a semiconductor laser which is widely utilized industrially has a remarkably long service life in comparison with a high brightness discharge lamp such as a high pressure mercury lamp. Also, since the semiconductor laser is a light source which is able to generate light of a single wavelength, a spectroscopic instrument such as a dichroic mirror is no longer required, so that there is an advantage that the entire apparatus can be made small-sized.
On the other hand, in a system in which a coherent light source such as a laser is used, a new problem of generation of speckles is brought about. The speckles form a spot-like pattern appearing when coherent light such as a laser light is irradiated on a diffusing surface, and is observed as spot-like unevenness in brightness, which is generated on a screen. The speckles become a factor which adversely physiologically affects an observer. The reason why speckles are generated when the coherent light is used is that the coherent lights reflected from respective parts of a diffusing reflection surface such as a screen have remarkably high coherency and the lights interfere with each other. For example, “Speckle Phenomena in Optics” by Joseph W. Goodman, Roberts & Company Publishers 2006, provides detailed theoretical consideration with respect to generation of speckles.
Thus, since a problem inherent to the system using a coherent light source, which is generation of speckles, is brought about in the system, a technology to control generation of speckles has been proposed. For example, Japanese Patent Publication No. 6-208089A(1994) discloses a technology in which laser light is irradiated to a scatter plate, the thus obtained scattered light is led to a spatial light modulator, and the scatter plate is driven and rotated by a motor, thereby reducing the speckles.
As described above, although, in a projection type image display apparatus using a coherent light source, a technology to reduce speckles has been proposed, it is not possible to efficiently and sufficiently control the speckles by means of conventionally proposed methods. For example, with the method disclosed in Japanese Patent Publication No. 6-208089A(1994), since laser light is irradiated onto a scatter plate and is scattered, a part of the laser light is wasted without contributing to image display. In addition, although it is necessary to rotate the scatter plate in order to reduce the speckles, such a mechanical rotation mechanism becomes a comparatively large apparatus, and power consumption is increased. Further, since the position of the optical axis of illumination light does not change even if the scatter plate is rotated, it is not possible to sufficiently control the speckles generated on the diffusing surface of a screen.